The production of circuit boards typically includes loading the boards onto a conveyor system and transferring them down a conveyor line where the leads of electrical components are inserted into the holes through the circuit board by a machine and/or manually. The boards are then transferred to an edge handling conveyor which transfers the boards through a flux station where solder flux is applied to the circuit boards so that the leads of the electrical components can later be soldered to the metallized areas of the board with a high quality solder bond. After the flux application station, the boards are transferred through a preheat zone to flash off the solvents from the flux and to preheat the board to minimize thermal shock from contact with solder wave. Continuing the process, the board is transported through a wave solder machine where the board moves over a wave of solder. The solder is drawn or forced up into through-holes containing the leads of the electrical components and solder bonds between the leads and the metallized sections of the board are formed. After leaving the wave solder machine, the board is sent through a cleaning machine to remove the residue left from the flux. The effectiveness of the flux application, the type of flux being applied, the need to clean the board subsequent to soldering, and the need to clean the coating chamber in which the solder flux is applied, each present problems which are addressed by the present invention.
Presently, three types of flux, i.e. rosin flux, organic acid flux, and low solids flux, are being used in the manufacture of circuit boards. Rosin flux and organic acid flux are the most prevalently used fluxes at this time. These two fluxes are normally applied by contacting the circuit board with either a liquid wave or a foam of flux, the latter being obtained by bubbling a gas through a flux which is made to foam. Alternatively, the flux can be applied to the circuit board by brushing or spraying. In both the liquid wave and foam application systems, an open vat or barrel containing the flux must be constantly monitored to keep the percent of solid to solvent within a range of predefined levels since solvents evaporate out of fluxes. The level of solvent content is determined by testing the specific gravity with an hydrometer. Whenever, the specific gravity moves out of the predefined range, a solvent, such as alcohol, is added to the open flux container to adjust the specific gravity as necessary. The amount of solvent needed to maintain the specific gravity in the required range and the constant monitoring requirement is a significant expense in the operation of the prior art systems.
While rosin-based fluxes dominate the electronic industry, there are serious problems associated with the use of these fluxes. Rosin-based fluxes often leave residues on the circuit board which detract from the quality of the board. For this reason, circuit boards using this type of flux must be cleaned after soldering. Moreover, since rosin residues are difficult to remove, harsh industrial detergents or chlorofluorocarbons must be used to clean the circuit board. The disposal of spent industrial detergents is usually expensive while the use of chlorofluorocarbons has been found detrimental to the environment. Because of these environmental concerns, chlorofluorocarbons are being phased out of use in the United States. They are becoming increasingly more expensive, as they are phased out. The cleaning equipment to apply the chlorofluorocarbons is required for each circuit board production line and is expensive to purchase, operate and provide the needed floor space.
The organic acid fluxes, on the other hand, can be effectively cleaned by water. However, water cleaning equipment is required for each circuit board production line and is expensive to purchase, operate and provide the needed floor space. The floor space requirement is of particular concern and expense because the length of the cleaning equipment.
In an effort to eliminate the need to clean the circuit boards after soldering, low-solid fluxes, or "no-clean fluxes", which contain small amounts, e.g., 1-5% by weight of solids (activator and vehicle) and the remainder liquid solvent, such as isopropyl alcohol, are being put increasingly more into use by circuit board manufacturers. Because of the small amount of solids within no-clean fluxes, the amount of residue left on the board is significantly reduced, as compared to the amount of residue remaining after the use of conventional rosin-based fluxes. These low-solid fluxes are particularly attractive because, as their name implies, cleaning of the circuit boards after soldering is not required, which is a tremendous cost savings. However, the application of the no-clean fluxes has been a problem. With the prior methods, i.e., using an open vat, the constant monitoring and control of the specific gravity has been difficult. Therefore, spraying no-clean fluxes with a closed system is the preferred method of application. The flux is stored in a closed container and delivered to a spray system with minimal prior exposure to air so that the solvents don't evaporate. The problem with the prior art spray systems is that they typically use a low pressure spray, which operates in the "air spray" fluid pressure ranges of 50 to 60 psi, and generates a lot of overspray within the coating chamber. This overspray wastes flux coating material and results in the chamber and the conveyor becoming covered with the flux coating material and being difficult and time consuming to clean. Also, the overspray tends to clog up the control sensors which signal the location of the circuit board to the control system. This is detrimental because the spray is frequently turned on when the board is not in the proper location. This exacerbates the problem by generating even more overspray and creating an even greater mess which requires more frequent cleaning. Whenever the system is cleaned, the entire line must be shutdown, further increasing the overall manufacturing costs of the circuit boards. Another problem with the low pressure spray is that the flux does not always adequately penetrate the through-holes containing the leads to be soldered. This results in less effective solder connections between the leads of the components and the circuitry carried on the board.
These same poor through-hole penetration and excess overspray conditions also occur when the low pressure spray method is utilized with the other two types of solder fluxes as well.
Therefore, there is a need for a technique of applying solder fluxes, and particularly low-solid fluxes, to circuit boards so that overspray is minimized, and the overspray that is produced is effectively removed from the coating chamber to prevent it from collecting on the coating chamber, the conveyor, and the control sensors, while uniformly applying solder flux to the board and achieving excellent through-hole penetration.